The present invention generally relates to a propulsion system wherein two high speed turboshaft engines are disposed side-by-side and employed to drive a single output shaft and, more particularly, to a ballistic shield containment system positioned between the engines to prevent rotor burst in one of the engines from adversely impacting the other turboshaft engine.
The principal drawback of a single engine aircraft resides in the almost inevitable result that engine failure will cause the aircraft to crash. For this reason, federal law has long prohibited the carrying of passengers for hire in single engine, propeller-driven fixed wing aircraft. Rather, commercial aircraft that carries passengers must utilize at least two separate engines, allowing one engine to continue to propel the aircraft even if the other engine should fail.
While multiple engines add a considerable and necessary level of safety to operation of the aircraft, they also significantly add to the cost of the aircraft. For small airlines, the expense of the aircraft including the expense of the separate engine assemblies can be the primary factor as to whether the airline can be commercially successful.
In an effort to provide an economical alternative to the need for aircraft employing completely separate engine assemblies, the concept has been put forth of employing dual aircraft engine power sections to drive a single propeller shaft through connections of the engine output shafts to a common gearbox. If one of the engine assemblies should fail, the remaining engine assembly will provide sufficient propulsion. Such a multi-engine, single propeller propulsion system is shown in Soloy, U.S. Pat. No. 4,829,850. The engines drive a single propeller and are separated by a firewall. There is no suggestion in Soloy of utilizing a containment system designed to withstand the serious type of engine failure referred to, by those skilled in the art, as rotor burst. Nor is there any teaching of a detailed attachment/suspension system capable of transmitting forces impacting against the firewall without transmitting any moment loads.
In rotor burst, the compressor or turbine disk breaks apart into portions, which may be as large as a third the size of the disk. These portions typically travel at speeds of up to 450 miles per hour and can easily breach conventional gas turbine engine firewalls. For example, the kinetic energy associated with a rotor burst of a first stage compressor impeller where the fragment is about ⅓ the size of the disk is on the order of 859,040 in-lbf. In comparison, a blade fragment from a first stage compressor impeller has a kinetic energy of about 28,719 in-lbf. For a second turbine stage, the burst fragment may have kinetic energy of about 406,770 in-lbf while the blade fragment 16,973 in-lbf. It is evident that the damage created by rotor burst has the potential for causing extreme damage and even complete failure to an adjacently disposed engine assembly regardless of the presence of a conventional firewall as taught in Soloy.
Because of the danger of fragments from one engine section damaging the remaining engine section, certification for the multi-engine, single propeller shaft design basically requires demonstration that failure of one engine section will not create a hazard to the remaining engine section.
As can be seen, in order to certify a two engine section assembly for commercial passenger use, there is a need for ballistic shield containment system that ensures continuous power is available from one of the two power sections in the event of an uncontained rotor failure in the other power section. Such a containment system must be able to withstand the impact of fragments generated by all types of engine failure up to, and including, rotor burst.
In one aspect of the present invention, a ballistic shield is formed as a substantially rectangular panel member having a uniform thickness of approximately xc2xd inch and preferably formed of metallic material possessing an optimal combination of strength and ductility and low weight density, for example titanium alloy Ti-6-4. The shield has a number of features which combine to resist fragment penetration and excessive deformation and displacement, on the order of 2-3 inches total deformation and displacement relative to the unaffected power section, caused by any type of structural failure including rotor blast. The features include a reinforcing lip mounted on the periphery of the shield which extends outwardly beyond each face of the panel member. In addition, reinforcing ribs are strategically located to extend outwardly from each face of the shield to reinforce specific panel areas where fragments may have the greatest force of impact. The ballistic shield extends in a generally vertical direction approximately equal distance between the two power sections on the same plane as the propeller shaft.
The ballistic shield is supported by a support truss system, which is attached to a multi-part space frame assembly. The space frame, in turn, is connected by a separate space truss system to fixed support assembly such as an aircraft""s forward bulkhead structure. All primary support joints between the ballistic shield and its supporting truss system incorporate spherical bearings to assure that while direct force loads are transmitted through the joints, there will be no transmission of moment loads. Secondary support joints of the ballistic shield are bolted directly to its supporting structure to provide a degree of dynamic frictional damping.
In another aspect of the invention, a turboprop engine assembly including multiple engine power sections driving a single propeller shaft is protected and supported by a multi-part space frame assembly. A ballistic shield is supported by the space frame at a location approximately mid-point between the engine power sections in the same plane as the single propeller shaft. A truss system connects the ballistic shield to the space frame and a separate truss system connects the space frame to the aircraft""s forward bulkhead.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.